Posttranscriptional gene silencing or RNA interference (RNAi) has been reported to be accompanied by the accumulation of small (20-25, e.g., 20, 21, 22 nucleotide) fragments of double stranded RNA, which are reported to be synthesized from an RNA template (Hamilton & Baulcombe, Science 286:950-952 (1999)). These fragments are called small interfering RNAs (siRNAs). It has become clear that in a range of organisms, including mammals, siRNA is an important component leading to gene silencing (Fire et al., Nature 391:806-811 (1998); Timmons & Fire, Nature 395:854 (1998); WO99/32619; Kennerdell & Carthew, Cell 95:1017-1026 (1998); Ngo et al., Proc. Nat'l Acad. Sci. USA 95:14687-14692 (1998); Waterhouse et al., Proc. Nat'l Acad. Sci. USA 95:13959-13964 (1998); WO99/53050; Cogoni & Macino, Nature 399:166-169 (1999); Lohmann et al., Dev. Biol. 214:211-214 (1999); Sanchez-Alvarado & Newmark, Proc. Nat'l Acad. Sci. USA 96:5049-5054 (1999); Elbashir et al., Nature 411:494-297 (2001)). As gene silencing is a powerful tool for regulation of gene expression, both of endogenous genes and of transgenes, improved methods of gene silencing and improved methods of making siRNA libraries are desired.